The invention is directed to structure for controlling overflow of liquid from a pressure vessel containing a volume of liquid metal with a free surface.
In the Clinch River Breeder Reactor (CRBR), sodium is employed as the liquid metal coolant and the coolant level must be maintained while providing the flexibility to add purified sodium, remove sodium for purification, and accommodate thermal expansion (swell) and contraction (shrink) of the sodium. Also the pressure vessel must be maintained below its design metal temperature of 900.degree. F.; the temperature of sodium leaving the reactor is 995.degree. F., and the normal operating temperature of main sodium pool is about 1045.degree. F. in the upper regions. The coolant level must be maintained within an acceptable range, since a high sodium level in the reactor would cause the following:
1. Flooding of the cover gas inlet and outlet nozzles.
2. Buildup of gas pressure in the cover gas system -- resulting in the loss of riser seals, etc.
3. Assuming that the rise in level is also associated with a rise in temperature, the sodium temperature would cause the reactor vessel upper assembly to exceed maximum allowable temperatures, resulting in an unacceptable condition to occur. It is unlikely that any safety function would be affected but the reactor would have to be shut down permanently since there is not any design criteria in the ASME Code for certain materials beyond certain temperature limits. The vessel itself could withstand the pressure rise associated with an extreme level increase.
A low level would cause inadequate cooling of the reactor core.
In a previous design for the CRBR, coolant level is controlled by several standpipes penetrating the vessel and a concentric thermal liner inside the vessel and having funnel-like terminations located within a gutter on the inner periphery of the thermal liner. The outlet ends of the standpipes are connected to a sodium overflow tank, on which a recirculation and purification system takes suction. Bypass coolant in a bypass annulus between the concentric thermal liner inside the pressure vessel passes into the gutter through weirs in the top of the thermal liner. The weirs are placed below the elevation of the openings in the standpipes. On the other side of the gutter a similar series of weirs, located at an elevation higher than the openings in the standpipes, connect the gutter to a reactor outlet plenum which is a mixing region above the reactor core containing the coolant heated by passages through the core to approximately 1045.degree. F. Coolant at 730.degree. F. enters the reactor pressure vessel via an inlet plenum below the core but a small fraction does not pass through the core and hence is not heated by nuclear fission; instead, this fraction (about 2.1 percent) of the flow is routed around the core by the core support structure, and enters the bypass annulus through holes in the thermal liner just above the juncture of the liner with the pressure vessel. Thus, the bypass coolant maintains the temperature of the pressure vessel shell below 900.degree. F. The elevation of the coolant level is highest in the bypass annulus, intermediate in the space between the weirs and lowest in the outlet plenum; the difference in coolant level elevation between the bypass annulus and outlet plenum results from the smaller resistance to flow through the bypass annulus. The bypass annulus coolant level rises until flow to the standpipes and into the outlet plenum together equal the coolant flow into the bypass annulus. The normal coolant level in the outlet plenum is at the elevation of the weirs on the inner periphery of the gutter. A constant portion of the bypass coolant with this arrangement is always removed from the pressure vessel via the standpipes and hence constant makeup coolant flow is required or else the coolant level in the outlet plenum would gradually decrease due to the net loss of coolant. Also, it is possible for reverse flow to occur from the outlet plenum to the bypass annulus in case of level transient; this is undesirable since the pressure vessel would be exposed to the 1045.degree. F. coolant in the outlet plenum. Furthermore, as presently designed, the gutter containing the standpipe and weirs extends far enough toward the vertical center line of the pressure vessel so that physical interference does occur with the control rod drives, supports, instrumentation, and handling equipment comprising the upper internal structure of the Clinch River Breeder Reactor.